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Main approaches for controlling the stiffness and damping of a structure
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Dr. N. Subramanian
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PostPosted: Fri Apr 28, 2023 4:19 pm    Post subject: Re: Main approaches for controlling the stiffness and damping of a structure Reply with quote

Replies by Er.  Ron Hamburger



Do you have a good resource for determining the modal response factors (i.e., mass participation) for a uniformly distributed system (i.e., a simple span beam)?


The ATC-40 publication, Seismic Evaluation and Retrofit of Buildings,
Chapter 8, gives a good overview of the computation of modal shape
factors and mass participation factors for structures having different types of structural framing systems. It suggests that for typical moment frame buildings, with relatively uniform drift ratio in all stories, the mass
participation factor is typically about 0.8. For cantilevered shear wall
structures, with larger story drift at upper levels, the factor drops to about 0.7. Differences occur of course, depending on the specifics of the mass and stiffness distribution.



For nonlinear seismic response, do we expect smaller force, but larger displacement compared to linear analysis?


The answer to this depends on both the strength and fundamental period of the structure. If a structure is sufficiently strong to respond to
earthquake shaking without damage, nonlinear analysis will not show
reduced forces. However, for most structures that are not designed to
respond elastically to ground motion, the forces predicted by nonlinear
analysis will be less than those predicted by linear analysis.

For structures having "short" period (i.e., on the horizontal plateau) the nonlinear displacement will be larger than the linear displacement. N.M. Newmark specified a rule of thumb that for such structures, the nonlinear displacement would be approximately the square root of (2μ-1) times the linear displacement. For longer period structures, the nonlinear displacement is about equal to the linear displacement.

Of course, if nonlinear behavior results in substantial additional damping, the nonlinear displacements will be less than these. The ASCE 41 standard attempts to capture these effects with the C1, C2, and C3 coefficients used to formulate the design base shear in that standard.
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Dr. N. Subramanian
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PostPosted: Fri Apr 28, 2023 4:22 pm    Post subject: Re: Main approaches for controlling the stiffness and damping of a structure Reply with quote

Reply by Er.  Ron Hamburger


Why are there two different earthquake levels defined: maximum considered earthquake, and design earthquake? What is the relationship between them?


This is because of the historic development of the building codes. Prior to the 1990s, the code specified a single earthquake level for design, called the design earthquake. It was defined as an "earthquake having the intensity of the worst earthquakes that had historically occurred in the region."

In the mid-1970s, the ATC 3-06 publication, a predecessor to the
NEHRP Recommended Seismic Regulations for Buildings and Other
Structures, and the basis of the current codes, suggested that the design earthquake had a return period of 475 years (10% probability of
exceedance in 50 years). In the 1990s, FEMA and USGS collaborated to
develop seismic provisions that would be nationally applicable, as the
provisions then in use in the building code were primarily developed for
the earthquake hazards and building practices in the western U.S.
(California, Oregon, Washington).

The national panel realized that because severe earthquakes occur less frequently in the eastern US than in the west, a 475-year return period was not adequate for design in the east.

The MCE (originally taken as having a 2,475-year return period) was
adopted because it captured the severe earthquakes that occur in places
like Charleston, South Carolina, and the New Madrid region of Tennessee
and Missouri. Since it was reasoned that most well-designed structures
could experience ground motion 50% stronger than the design level,
without collapse, it was decided to redefine the Design Earthquake as
1/150% (2/3) of the MCE. The primary goal of the building code evolved
from providing "Life Safety" at the Design Earthquake to one of providing "Collapse Prevention" for the MCE.
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Dr. N. Subramanian
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PostPosted: Fri Apr 28, 2023 4:26 pm    Post subject: Re: Main approaches for controlling the stiffness and damping of a structure Reply with quote

Replies by Er.  Ron Hamburger


Is there any way to dissipate energy in flat plate with shear wall system during the earthquake? Can you put some wall with less stiffness than other to dissipate energy?


Any structural element that undergoes inelastic straining in response to
earthquake shaking will dissipate energy. In shear wall structures this is
typically achieved by designing the walls such that the bases of the walls
can develop plastic hinges in flexure and the coupling beams between wall piers can develop flexural hinging as well.

The textbook by Dr. Moehle entitled "Seismic Design of Reinforced Concrete Buildings" describes this well as does the NIST Tech brief, Seismic Design of Special Reinforced Concrete Walls, Report No. GCR-14-917-31. The NIST document is available for free online.



Did the ground motions go up from ASCE 7-16 to 7-22, particularly in Los Angeles and California?

The ground motions in many areas of the U.S. including portions of
California did go up from ASCE 7-16 to ASCE 7-22. There are two reasons for this. First, the seismic hazard values provided by the USGS for ASCE 7-22 now include consideration of basin effects, whereas ASCE 7-16 ground motions did not.

Basin effects occur where relatively shallow "basins" of soft soils overly bedrock. Essentially, the seismic waves from the rock enter these basins and reflect repeatedly within the basin, amplifying motion intensity. Much of Los Angeles lies in such a basin. In addition, the spectral shape of ground motions on soft soil sites subject to large magnitude earthquakes, as reflected in the multi-period response spectra of ASCE 7-22 have larger amplitudes at periods of 1 to 3 seconds, than do the spectra of ASCE 7-16.
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Dr. N. Subramanian
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PostPosted: Fri Apr 28, 2023 4:29 pm    Post subject: Re: Main approaches for controlling the stiffness and damping of a structure Reply with quote

Replies by Er.  Ron Hamburger


What are acceptable methods to determine shear wave velocity of soils in upper 100 feet?


There are several methods for doing this. Some involve boring holes into
the ground and setting of small charges to measure the transmission time of sound through the soil. This can be done in single holes (so-called downhole measurements) or between multiple holes (so-called cross hole measurements). There are also methods of measuring this using sonar type equipment in which sound is "injected" into the soil at the surface and the time for it to reflect off different layers of soil and come back to the surface is measured.


What are your thoughts on the use of quenched-tempered (QT) bars on seismic design of beams and columns?


AISC 341, Seismic Provisions for Steel Structures, sets the permissible
grades of steel for use in seismic force resisting systems. ASTM A913, a
quenched and tempered grade of steel for wide flange members is
specifically permitted by that specification. The specification does not
specifically permit the use of Q&T steel bars. Having said this, Q&T steels typically have excellent strength as well as ductility. It should be possible to use such steels, if performance-based approaches are adopted. This would no doubt entail some level of laboratory testing to demonstrate that members fabricated from such steel will perform adequately.

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jaswant_n_arlekar
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PostPosted: Wed May 29, 2024 2:59 pm    Post subject: Re: Main approaches for controlling the stiffness and damping of a structure Reply with quote


Thank you, Dr. Subramanian for posting this.

The 26 April 2023 Wide Circulation of IS 1893 Draft Indian Standard Criteria for Earthquake Resistant Design of Structures Part 1 General Provisions [Seventh Revision of IS 1893 (Part 1)] is proposing to include an updated acceleration response spectrum based by Site Class (from A to E), corresponding to the Weighted Shear Wave Velocity. The existing version of the code has acceleration response spectrum for 3 soil types, hard, medium and soft.

The weighted shear wave velocity Vs is to be obtained by geotechnical investigations of soil up to the depth of influence or 30m. Cl. 6.2.4.1 of the WC draft IS 1893(1):April 2023 gives the expression for the same.

This means that geotechnical investigations will have to include Vs with the other parameters.
But the draft also includes an expression for Vs using the corrected STP Ni values.

Conducting tests like MSWA that give the Vs may be costly, and the availability of a method of calculating Vs using corrected N values will be used by consultants. But in the long run, it may be a good idea to have a correlation between the various methods of obtaining Vs.

Thank you, and with best regards,
Jaswant N. Arlekar

Dr. N. Subramanian wrote:
Replies by Er.  Ron Hamburger


What are acceptable methods to determine shear wave velocity of soils in upper 100 feet?


There are several methods for doing this. Some involve boring holes into
the ground and setting of small charges to measure the transmission time of sound through the soil. This can be done in single holes (so-called downhole measurements) or between multiple holes (so-called cross hole measurements). There are also methods of measuring this using sonar type equipment in which sound is "injected" into the soil at the surface and the time for it to reflect off different layers of soil and come back to the surface is measured.


What are your thoughts on the use of quenched-tempered (QT) bars on seismic design of beams and columns?


AISC 341, Seismic Provisions for Steel Structures, sets the permissible
grades of steel for use in seismic force resisting systems. ASTM A913, a
quenched and tempered grade of steel for wide flange members is
specifically permitted by that specification. The specification does not
specifically permit the use of Q&T steel bars. Having said this, Q&T steels typically have excellent strength as well as ductility. It should be possible to use such steels, if performance-based approaches are adopted. This would no doubt entail some level of laboratory testing to demonstrate that members fabricated from such steel will perform adequately.

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alpa_sheth
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PostPosted: Fri May 31, 2024 11:57 am    Post subject: Re: Main approaches for controlling the stiffness and damping of a structure Reply with quote

Hi Jaswant,

I had got some MSwA tests carried out in a Zone V (soon to be Zone VI) site a few years ago. The cost of the tests is not expensive, in fact it may be cheaper than conventional SPT tests. The problem is about the reliability of the results. Choosing an agency judiciously is very very important.

I am wary of unvalidated tests that depend only on electronic gadgets and we have no method of verifying the results. So for a few years we will need to validate the Vs obtained from MSwA tests with what we get from  equations using conventional SPT tests.

***************************************

@ Dr Subramanian- Thanks much for sharing the response of Er Ron Hamburger. Your attendance and knowledge sharing approach  benefits all of SEFI!!


best regards,
Alpa


[quote="jaswant_n_arlekar"]
Thank you, Dr. Subramanian for posting this.

The 26 April 2023 Wide Circulation of IS 1893 Draft Indian Standard Criteria for Earthquake Resistant Design of Structures Part 1 General Provisions [Seventh Revision of IS 1893 (Part 1)] is proposing to include an updated acceleration response spectrum based by Site Class (from A to E), corresponding to the Weighted Shear Wave Velocity. The existing version of the code has acceleration response spectrum for 3 soil types, hard, medium and soft.
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